The present invention relates to the field of electronic device manufacturing, and more particularly to a method and apparatus for precisely positioning the leads of surface mountable electronic components on the pads of a printed circuit board.
When large numbers of electronic devices are manufactured commercially, it is usually not economical to manually solder the individual leads of each electronic component to the corresponding pads of the printed circuit boards. Instead, commercial manufacturers usually use one or more techniques to simultaneously solder the leads of many components to the printed circuit board pads. Such techniques increase the speed and efficiency with which the devices are manufactured, and hence reduce costs.
One such cost saving technique uses surface mountable components. With this technique, the pads of a printed circuit board are covered with solder paste using the well known silk screening process. Solder paste is a finely divided mixture of solder, flux and solvents. The silk screening process coats each pad with a thin layer of solder paste while at the same time leaving the remainder of the board free of the solder paste. Then, the leads of each electronic component are placed on the appropriate solder paste coated pads of the printed circuit board. The printed circuit board, with the electronic components resting on the solder paste coated pads, is then moved into a furnace to melt the solder paste. As the temperature rises, the flux component of the solder paste melts first. The molten flux cleans the surface of the pads and the component leads to any accumulated dirt, oxidation and grease. Further increases in temperature melt the solder component of the solder paste. The molten solder flows over the leads to create an electrical connection between the electronic components and the pads. When the printed circuit board is removed from the furnace, the solder resolidifies, and thereby establishes both an electrical and a mechanical connection between the printed circuit board and the electronic components.
To produce a high quality solder joints between the pads and the leads, it is important to center the leads on the pads so as to produce the greatest area of contact between the pads and the leads. It is also important for esthetic reasons to center the leads on the pads. The pads of a printed circuit board are frequently positioned in straight columns and rows. Thus, if all the component leads are properly centered on the pads, the components will also be positioned neatly in straight columns and rows. After a board is assembled, even slight deviations in the alignment of the various components becomes quite noticeable. Thus, unless the component leads are centered on the pads, the workmanship looks sloppy and a customer may infer that the board is poorly constructed, even if the electrical quality of all the solder connections is actually good.
Electronic device manufacturers have recognized the importance of properly centering electronic components on printed circuit board pads. As a result, the industry has developed electronic component handling devices, called pick and place machines, which can position electronic components on a printed circuit board to within .+-.0.0005 inch. However, these pick and place machines only form one part of a device assembly line. Thus, as the components are transported along the assembly line from the pick and place machine to the next work station on the assembly line, the boards may be jostled or subjected to vibrations which can knock the components from the center of the pads. To combat this problem, the industry has also developed ultra smooth component conveyor systems to smoothly transport printed circuit boards from the pick and place machine to and through the furnace. However, the precision machining and design required to build such a conveyor increases the cost of these systems.